A purchase ordering system can be evaluated against a long requirements list or against five capabilities that decide whether it works at all. The five are cheap in any product, expensive to add afterwards, and between them they support matching, accruals and delivery chasing. Everything else is preference or scale.
Numbering and approval
Unique numbers issued automatically from one source and never reused, because a reused number makes two transactions permanently indistinguishable. And approval before the order goes to the supplier, since the commitment exists once they act on it. These two are the control; the rest is record keeping.
Line-level receipts and dated revisions
Ordered, received and invoiced quantities held per line, so partial deliveries are representable and the accrual comes free. And revisions kept with their dates rather than overwriting, so what was agreed when is answerable. Products missing either are document generators wearing a system's name.
A readable export
Orders, lines, receipts, revisions and approvals, in a format you can open without the vendor. Organisations change these systems more often than they plan to, usually because they outgrow the first, and an export that omits receipt history makes that move considerably worse. Test it in a trial rather than accepting a description.
Questions people ask about purchase ordering system
Is our accounting package enough?
Sometimes. Check line-level receipting and revisions specifically, since those are the common gaps rather than the ordering.
What about services?
They need the same shape, with the receipt being confirmation that work was done. Services are where untracked commitments accumulate.
When should we upgrade?
When somebody starts a spreadsheet alongside the system to track what is outstanding.